The present invention evolved from continuing development efforts pertaining to subject matter disclosed in my co-pending Application Ser. No. 265,128 filed May 19, 1981, entitled "Symmetric Microwave Mixer". This co-pending Application discloses a diode mixer with compact, symmetric conductor port structure significantly reducing unwanted sensitivity to adverse parameters.
The present invention relates to improvements in the microstrip circuit layout and structure in those applications where greater isolation is desired. Each port to the mixer diodes is provided by balun circuitry. The mixer diodes are connected solely to the secondary circuits of the baluns and are isolated from the primary circuits of the baluns.
In an exemplary down converter, a first balun supplies the RF port to the diode mixer, and one or two dual baluns supply the LO ports, for double or single sideband operation, respectively. Each dual balun has two pairs of balanced secondary conductors juxtaposed a primary conductor. The field between each secondary pair is applied across a separate diode of the mixer. The first conductors of the pairs are connected to a respective one of the balanced secondary conductors of the first RF balun and are separated by the same gap. The second conductors of the pairs are interconnected in common to a point between the diodes.
In a desirable aspect of the invention, coplanar diode mounting is achieved without sacrificing compactness or symmetry of the mixer port structure. This coplanar diode connection eliminates any need to extend one or more diode leads through the substrate. This latter type of through-lead connection may be cost objectionable in certain applications. The microstrip circuit layout of the present invention does not require holes in the substrate to complete the mixer circuit or to mount the diodes.
In another desirable aspect of the invention, IF voltages are not present on the RF or LO conductors, whereby to eliminate the need for filters to remove IF signals from the RF or LO signals. The IF voltages are also removed at a low RF potential point obviating the need to remove RF from the IF voltages and vice versa.